The present invention relates to mass measuring processes and apparatus, and more particularly, to mass sensing devices capable of real-time measurement of the minute quantities of material deposited on the surface of a test sample prior to deposition forming of semiconductor chips or the like, by means of signals conducted to sensing and evaluation circuitry positioned external to the deposition chamber. Sensor arrays built up from many such mass sensing devices permit real-time sensing of deposition patterns over the surface of such an array.
Over the past four decades, techniques have been developed for measuring small masses by depositing the mass on the surface of a quartz crystal oscillator and noting the resulting decrease in the frequency of the crystal. These devices are commonly referred to as quartz crystal microbalances ("QCM"s) and comprise a thin quartz crystal sandwiched between two metal electrodes. When an alternating electric field is applied across the piezoelectric quartz crystal, a shear-induced acoustic wave is generated whose frequency is sensitive to changes in mass. This enables small masses to be quantified with a sensitivity on the order of 10.sup.-9 grams/cm.sup.2 per Hertz frequency shift.
Typical QCM elements have areas in the neighborhood of 1 cm.sup.2 with excitation frequencies on the order of 10.sup.7 Hz. However, QCM instrument packages are commercially available that are as small as a person's thumb. Although QCMs are capable of measuring extremely small quantities of matter and are very portable, some disadvantages of this existing technology include its susceptibility to thermal gradient and mass gradient-induced errors. Therefore, corrections must be applied to QCM data whenever the devices are unevenly heated or mass is non-uniformly deposited on active QCM surfaces.
Other kinds of devices currently exist that are also capable of measuring small masses. One of these devices has been developed more recently and is known as a tapered element oscillating microbalance ("TEOM"). Although this instrument also interprets a mass change as a function of a frequency shift, important differences exist between QCMs and TEOMs. Whereas QCMs measure small amounts of matter deposited on a surface, the TEOM is primarily intended to measure the concentration of solid particles in a sampled gas stream. Also, in contrast with QCM which relies on a piezoelectric effect to generate its vibratory motion, the TEOM is driven electromagnetically. In the basic TEOM embodiment, a gas is pumped through the end of the hollow tapered element at a known flow rate. Solid particles are then removed from the gas flow by a filter and the subsequent mass change results in a frequency shift.
Like QCMs, TEOMs are not without problems. The oscillation of the tapered element in TEOMs is controlled by applying an alternating voltage bias to the body of the tapered element. The rate at which this voltage alternates, and hence the rate at which the TEOM vibrates, is controlled by a feedback circuit employing an LED/phototransistor combination that produces an AC voltage based on the motion of the tapered element. Therefore the signal produced by the phototransistor serves as a drive voltage which self-adjusts to alternate at the resonant frequency of the tapered element. The frequency of this TEOM signal can be directly related to the mass accumulated at the free end of the device.
The sensitivity of commercially available TEOMs is on the order of 10.sup.-6 grams. However specialized TEOMs, such as those specially developed for NASA/Goddard Space Flight Center, have been operated with resolutions on the order of 10.sup.-12 grams under extremely well controlled conditions.
With regard to the fabrication of micromechanical devices, micromachining has been extensively practiced in the electronics industry using electron, ion, and X-ray bombardment. These techniques have been used in the fabrication of structures as small as a fraction of a micron long with diameters below 100 angstroms. Tiny vibrating bridges have also been constructed as a tool for fundamental research. These bridges however, do not oscillate under the influence of a controlled electric field nor do they generate an electrical signal.
U.S. Pat. No. 3,492,858 to Heflinger, et al. discloses a microbalance apparatus with a vibratory Elinvar reed located over a vibratory main frame, which is caused to vibrate by a signal applied to a bimorph crystal. A small pickoff coil senses the frequency of vibration of the Elinvar reed and feeds it back through a closed loop circuit causing the driver, frame, and reed to vibrate at the resonant frequency of the reed. Placement of a small mass or particle on the end of the reed alters the reed's resonant frequency according to the mass of the particle, permitting the mass of the particle to be determined. In the Heflinger system, the vibration of the reed is caused by a crystal driver which is not directly coupled to the frame. The Heflinger system however, is not suitable for integration to a silicon chip carrier, and is susceptible to thermal errors.
U.S. Pat. No. 3,926,271, to Pataschnick, discloses a microbalance which implements a thin walled quartz tube that has a tapered vibrating section. The vibrating section is preferably a hollow tube, but may alternatively comprise a solid rod. The tube is anchored at one end to a base, and at the distal end it is free to oscillate. The tapered element is driven by alternating fields generated by reaction between alternating currents passed through leads and a field generated by the potential between two electrodes. The vibrating section preferentially vibrates at a given resonant frequency. However, deposits on a substrate mounted on the distal end of the reed change the resonant frequency. This change in frequency is measured by electrical feedback as the frequency of the applied voltage is timed, or by a motion detector utilizing an optical transducer and light source. The Pataschnick microbalance is of relatively large size and is quite fragile, and would be unacceptable for applications with high inertial loads.
U.S. Pat. No. 4,429,574, to Barry discloses a mass measuring system which determines the mass of relatively larger objects by utilizing a vibrator for inducing vibration within a test object. A probe is connected to piezoelectric transducers for sensing changes in the resonant frequency of vibration and determining differences in mass of the object by evaluating such changes. The Barry mass measuring system is likewise inapplicable to high inertial loads and integration to a silicon chip carrier, due to its size and fragility. Further, the analysis of the Barry system performance requires solution of a series of relatively complex mathematical equations.
Thus, previously available microbalances all suffer from one or more disadvantages, including high cost, poor sensitivity, susceptibility to thermal errors, and lack of durability. The present invention overcomes the deficiencies of the prior art.